Monitoring cerebral ischemia using cerebral oximetry: pros and cons

.The near-infrared light with wavelengths of 700 to 1000 nm can penetrate the scalp and skull, and light up brain tissue in both adults and children. Near-infrared spectroscopy (NIRS) based cerebral oximetry measures the ratio of oxygenated hemoglobin over the total hemoglobin (sum of oxygenated and deoxygenated hemoglobin) at a depth of around 2 cm below the scalp in the part of the brain being illuminated, assuming the distance between the light emitter and detector is 4 cm. It is believed that venous blood contributes on average 70% to 75% of the volume of the total blood in the measured brain tissue in the frontal lobe. However, different individuals may have different percentages of venous blood in the frontal lobe, which explains why regional cerebral tissue oxygen saturation (rScO2 )a s measured using cerebral oximetry shows inter-individual variability [7] . The two essential determinants of regional cerebral tissue oxygen saturation (rScO2 )a re the cerebral metabolic rate of oxygen (CMRO2) (consumption) and the oxygen delivery to the brain (supply) [7]